Combining light sources of different spectra permit lighting devices to emit a light spectrum of almost any desired energy content. For example, red light can be combined with unsaturated green light to yield a light spectrum that renders colors similar to daylight or similar to incandescence depending on the amount of accompanying blue light. Using red, green, and blue light sources, colors from such sources can be combined in any proportion to yield any aggregate color within the gamut of colors.
Color is the visual effect that is caused by the spectral composition of the light emitted, transmitted, or reflected by objects. Human vision is primarily related to color and brightness (contrast) of the light source, and (if reflected light is present) the spectrum that is reflected from an object being illuminated.
As a heated object becomes incandescent, it first glows reddish, then yellowish, then white, and finally bluish. Thus, apparent colors of incandescing materials are directly related to their actual temperature (in Kelvin (K)). Practical materials that incandesce are said to have correlated color temperature (CCT) values that are directly related to color temperatures of blackbody sources. CCT is measured in Kelvin (K) and defined by the Illuminating Engineering Society of North America (IESNA) as “the absolute temperature of a blackbody whose chromaticity most nearly resembles that of the light source.” Light having a CCT below 3200K is yellowish white in character and is generally considered to be warm white light, whereas light between having a CCT between 3200K and 4000K is generally considered to be neutral white light, and light having a CCT above 4000K is bluish white in character and generally considered to be cool white light.
It is important that lighting be of appropriate intensity for the task at hand and also have appropriate color rendering characteristics. For most daytime tasks, light sources (whether artificial or natural) should have high intensity and high color rendering. Conversely, for sleeping, light should have very low levels. The color differentiation of night vision is very low.
Light affects circadian rhythms. Human physiology responds non-visually to the presence or absence of certain wavelengths. For example, blue light is known to suppress melatonin, and ultraviolet rays are known to damage the skin. The intensity of light and the spectral content of light have a strong effect on the human circadian rhythms. These circadian rhythms are ideally synchronized with the natural light.
Circadian rhythm disorders may be associated with change in nocturnal activity (e.g., nighttime shift workers), change in longitude (e.g., jet lag), and/or seasonal change in light duration (e.g., seasonal affective disorder, with symptoms including depression). In 2007, the World Health Organization named late night shift work as a probable cancer-causing agent. Melatonin is an anti-oxidant and suppressant of tumor development; accordingly, interference with melatonin levels may increase the likelihood of developing cancer. Methods involving stimulus with artificial light sources to modify the phase and amplitude of a human circadian cycle (e.g., for resetting purposes) have been developed, such as disclosed in U.S. Patent Application Publication No. 2006/0106437A1 to Czeisler et al.
Artificial light sometimes includes too much blue light in the evening, which suppresses melatonin and hinders restful sleep. It is principally blue light (e.g., including blue light at a peak wavelength value between 460 to 480 nm, with some activity from about 360 nm to about 600 nm), that suppresses melatonin and synchronizes the circadian clock, proportional to the light intensity and length of exposure. Exposure to artificial light during the night may inhibit a person from falling to sleep or returning to sleep, and may also cause a temporary loss of night vision.
Natural light varies with respect to intensity and/or color temperature depending on the season, latitude, altitude, time of day, and weather conditions. Natural light variation due to season and geographic location may be understood with reference to FIG. 1, which plots hours of daylight per day as a function of latitude and time of year. Natural light also varies each day with respect to intensity and color temperature. The changing color temperature of sunlight over the course of the day is mainly a result of scattering of light, rather than changes in black-body radiation. Ignoring variations due to weather conditions, natural light intensity typically is low at sunrise, increases through mid-morning to a high level at mid-day, and then decreases in mid-afternoon to evening to a low level at sunset. Color temperature also varies in a predicable manner. During sunrise and sunset, color temperature tends to be around 2,000K; an intermediate CCT value of around 3,500K is exhibited shortly after sunrise or before sunset (when daylight is redder and softer compared to when the Sun is higher in the sky); and a color temperature of around 5,400K is exhibited around noontime. Color temperatures for various daylight sources are tabulated in FIG. 2. Low (or warm) color temperatures are consistent with reduced blue content, while higher (or cool) color temperatures are consistent with increased blue content.
Generally, a light that is dim and exhibits a low (warm) color temperature promotes restfulness (e.g., such as may be desirable in the evening and night before sleep), and a light that is bright and exhibits a high (cool) color temperature promotes alertness (such as may be desirable in the morning and during the day). A light having a very low intensity and a very low color temperature would least interfere with a person returning to sleep after being awakened in the middle of the night.
Color changing lights are known in the art. One example of a color changing light bulb is the Philips “Hue” bulb (Koninklijke Philips N. V., Eindhoven, the Netherlands). Such bulbs permit different colors, color temperatures, and/or intensities of light to be selected by a user via a computer or portable electronic device.
Despite the availability of color changing lights, it can be difficult for users to program lighting devices to obtain desired illumination conditions that take into account variations in natural light that may be attributable to multiple factors such as the season, latitude, time of day, and weather conditions.
It would be desirable to provide lighting devices and methods that address limitations of conventional lighting devices and methods.